The present invention relates to the field of protecting the occupants of a motor vehicle using an inflatable airbag. More specifically, the invention relates to a hybrid gas generator of tubular shape for inflating such an airbag.
In order to inflate an airbag to protect the occupants of a motor vehicle, use has been made, in the past, of pressurized gas reservoirs opened by a pyrotechnic valve. Thus, U.S. Pat. No. 3,690,695 describes a device for inflating a protective airbag. This device consists of a reservoir of gas under pressure which is isolated from the duct leading to the airbag by a gastight diaphragm against which a finned solid piston rests.
In the event of a collision, a pyrotechnic primer placed behind the piston causes the piston to move forward, tear the diaphragm and allow the pressurized cold gases to enter the said duct. As the hot gases resulting from the combustion of the pyrotechnic charge cannot mix with the cold gases, these are heated up by passing over a bed of magnesium which reacts with them. A device such as this is dangerous in terms of its principle of operation, and has to be produced in a complex way.
Attempts have therefore been made at developing hybrid gas generators containing, on the one hand, a reservoir of cold gases under pressure and, on the other hand, a pyrotechnic charge capable of fulfilling two functions: opening the gas reservoir and heating the cold gases.
PCT application WO98/09850 thus describes a hybrid generator of tubular shape containing a pressurized gas and a piston coupled to a pyrotechnic initiator. In the event of a collision, the triggering of the initiator causes the piston to move and open the generator at its opposite end to the pyrotechnic initiator and also causes the cold gases to heat up by mixing, inside the generator, of these gases with the hot gases supplied by the pyrotechnic initiator. As this takes place, an abrupt rise in pressure inside the generator is observed and, for obvious safety reasons, the amount of gas that can be stored in the generator is limited to well below its theoretical maximum value. Finally, a generator in which the stroke of the piston has to be guided along the entire length of the generator is relatively expensive to produce.
To simplify the actual production of the generator, there has therefore been proposed, for example in PCT application WO98/12078, a type of tubular hybrid generator with lateral gas outlet orifices, the reservoir for pressurized cold gases being located on one side of these orifices, and the pyrotechnic chamber being located on the other side. The gas reservoir is opened by a hollow piston which has a central duct which, after opening, allows the hot gases to enter the reservoir to mix with the cold gases. The heated mixture then leaves the reservoir via the gap around the piston resulting from the breakage of the diaphragm which used to close the reservoir.
Although this solution does indeed yield tubular hybrid generators which are relatively simple to produce, it does not do away with the drawbacks that result from the fact that the hot gases from the combustion of the pyrotechnic charge enter the inside of the reservoir for pressurized cold gases.
This type of drawback is seen also in the solutions which, for opening the reservoir, use a projectile instead of a piston, as described, for example, in U.S. Pat. No. 5,464,247, or again in patent JP 10 250 525, or in those which use a piston with a shaped point as described, for example, in patent DE 19 545 077.
Tubular hybrid generators are particularly wanted for inflating front or side airbags for protecting the passengers of motor vehicles, but those skilled in the art do not currently have such tubular hybrid generators available to them which, simultaneously, are simple to produce and perform all of the mixing of the hot gases originating from the combustion of the pyrotechnic charge with the cold gases stored in the reservoir, outside the latter.
The object of the present invention is precisely to provide such a generator.
The invention therefore relates to a hybrid gas generator comprising a tubular body with an upstream end closed by a pyrotechnic primer and hot-gas-generating device and a downstream end which is closed in a gastight manner, the said tubular body also having gas outlet orifices and containing an internal partition with a central orifice of cross section S closed by a diaphragm, the said partition dividing the said tubular body into two parts:
an upstream part forming a combustion and mixing chamber and containing the pyrotechnic device and the gas outlet orifices,
a downstream part forming a reservoir chamber containing at least one presurized gas,
characterized in that the said tubular body consists of two cylindrical tubes joined together by one of their ends, the said partition being of one piece with one of the two tubes, and in that the said tubular body thus formed, in its upstream part, between the pyrotechnic device and the internal partition, contains a support piece which is fixed to the said body without coming into contact with the said internal partition or with the said gas outlet orifices and which contains a solid mobile piston consisting of at least one column of cross section s, smaller than the cross section S, which rests against the diaphragm, closing the central orifice of the internal partition, the said column, at its opposite end to the said partition, having a solid base of cross section So larger than S and which bears peripheral ribs preventing the said base from coming into contact with the said central orifice.
It is the particular structure of the generator, in which the internal partition is of one piece with one of the twoltubes that make up the body of the generator which, together with the use of a mobile piston which bears peripheral ribs that prevent the central orifice of the said partition from being blocked off, makes it possible to achieve the objective of the invention. Specifically, once the reservoir chamber has been opened by the column of the mobile piston, the cold gases can leave the reservoir chamber via that part of the central orifice of the internal partition which is left uncovered by the column of the piston to enter the combustion and mixing chamber. At the same time, the widened base of the piston acts as a deflector for the hot gases which originate from the pyrotechnic device. These hot gases cannot enter the reservoir chamber but are made to mix with the cold gases leaving this chamber so as to constitute the gaseous mixture which leaves the generator through its outlet orifices, the structure of the body of the generator giving perfectly reliable operation.
According to a first preferred embodiment of the invention, the said support piece and the said solid mobile piston isolate the said pyrotechnic device from the gas outlet orifices.
This embodiment makes it possible to ensure good protection of the pyrotechnic device and thus good preservation of the generator over time.
According to a second preferred embodiment of the invention, the said support piece consists of a hollow ring crimped against the body of the generator, the said ring having a central cylindrical recess which has an internal shoulder so that it has a diameter d1 facing the pyrotechnic device and a diameter d2 facing the internal partition, d2 being greater than d1, the said ring being extended, facing the internal partition, by a hollow cylindrical neck, the inside diameter of which is equal to d2, and the outside diameter of which is smaller than the inside diameter of the tubular body.
In this case, the said solid mobile piston advantageously consists, ion the one hand, of a cylindrical body which has a cylindrical shank of outside diameter d1, a cylindrical base of outside diameter d3 between d1 and d2 and a cylindrical column of cross section s and of length l and, on the other hand, of fins of height h smaller than the length l which surround the column and which rest on the base of the said cylindrical body, the said fins constituting the peripheral ribs.
In this embodiment, the base of the mobile piston rests on the internal shoulder exhibited by the support ring and the shank of the piston, the height of which has to be less than the stroke of the piston, enters that part of the central recess of the ring which faces the pyrotechnic device. Once the pyrotechnic device has fired, the hot gases begin to drive the solid mobile piston without being able to mix with the cold gases originating from the reservoir chamber. This then yields a hybrid generator which causes the protective airbag to start to deploy using only cold gases.
According to a third preferred embodiment of the invention, the said fins constitute a single piece exhibiting a cylindrical central duct of cross section s, the said piece being force fitted onto the said column of the cylindrical body of the solid mobile piston.
Advantageously, the cylindrical body of the piston will be a metal body and the said single piece will be made of a rigid plastic.
Finally, according to a fourth preferred embodiment of the invention, the two cylindrical tubes that make up the tubular body of the generator have identical outside and inside diameters and are joined together by welding. Advantageously, the internal partition will be of one piece with the tube that forms the upstream part of the generator. This last embodiment actually allows for particularly simple and reliable assembly of the generator according to the invention as will be explained in detail later on in the description.
The invention thus provides a hybrid tubular generator which is simple and inexpensive to assemble. This generator makes it possible to mix the hot gases and the cold gases while at the same time preventing the hot gases from entering the reserve of cold gases. In a given reservoir-chamber volume, it is therefore possible to compress a greater amount of gas than can be stored in a hybrid generator with the same characteristics but which does not exhibit this safety feature.
This possibility is enhanced by the fact that, prior to pyrotechnic operation, the column of the mobile piston rests against the diaphragm which closes the reservoir chamber and acts, with respect to this diaphragm, as a mechanical pillar improving its resistance to pressure.